Transversal filter functions generally are performed electronically using computers and specialized integrated circuits. The maximum signal bandwidth electronic filters can handle is about 100 MHz. Later developed, electronic tapped delay lines rely on surface acoustic wave devices and charge coupled devices or active charge transport devices. These later devices are limited to bandwidths of about 1 GHz or less and are not reconfigurable. Optical tapped delay-line filters have been fabricated with fixed delays and fixed tap weights. The fiber optic transversal filters have demonstrated capabilities of up to 15 GHz bandwidths (limiting factor varies) but they have fixed (not always, but mostly, unity) tap weights and are not reconfigurable.
The fiber optic transversal filters can handle higher bandwidth signals and faster tap weight update rates than any electronic computing technology. The improvement over other optical techniques is that the filter is reconfigurable. Such a fiber optic delay-line filter is shown in the U.S. Pat. No. 4,128,759 issued to Barry R. Hunt et al. Variable weighted taps in the range of 0 to +1 are operatively associated with different lengths of optical fibers to provide an electrical output signal which is a function of the sum of the incident light energy signals. As a consequence, higher bandwidth data may be processed than was previously possible and several filtering functions are conceivable such as beam forming. Independently, operative attenuators provide only a positive weighting which selectively varies the positive amplitude of the light energy signals propagating through each of the fiber optic delay-lines.
Thus a continuing need exists in the state of the art for an optical transversal filter having variable positive and negative weight capability which assures a large bandwidth (approximately 10) GHz processing capability.